This invention relates generally to gas turbine engines and, more particularly, to a method of accurately sensing the fan discharge total pressure in a fan duct having outlet guide vanes disposed therein.
In the control of turbomachinery, where certain operational parameters are sensed and changes made to bring about a desired performance, it is often required to sense mainstream pressure at a position behind an obstruction in the mainstream flow such as, for example, turbomachinery stators in the turbine area, or outlet guide vanes in the fan duct area. This is true because the flow field upstream of the obstruction may be too complex for purposes of sensing operational conditions. If the total pressure is being sensed, it is important to have a consistent mainstream pressure measurement, free from the effect of wakes or boundary layers. Consistent acquisition of this pressure with ordinary probes becomes difficult because the local low pressure regions (wakes or boundary layers) wander with operating conditions. In addition, hardware manufacturing tolerances may also cause variations of wake locations. It is therefore difficult, if not impossible, to predict the exact location where a sensor can best be placed within the mainstream in order to avoid the flow disturbance from the hardware.
In particular, one method of maintaining the fan operating line of a turbofan engine is by regulating the exhaust nozzle so as to maintain a constant value of [fan discharge total pressure (P.sub.t) - fan discharge static pressure (P.sub.s) / fan discharge static pressure (P.sub.s)] or .DELTA.P/P. Since the fan discharge total pressure, P.sub.t, is necessarily sensed at a location behind the outlet guide vane in the fan duct, the wakes created by the outlet guide vane tend to cause a low, nonrepresentative P.sub.t acquisition, with the resultant nozzle opening lowering the fan discharge pressure below the level desired. This induced error becomes more predominant at higher speeds when the wake becomes more pronounced. Because of slight structural differences between engines, the location of these pressure defects with respect to the probes could vary from engine to engine. Further, since the characteristic flow around the outlet guide vanes will vary with engine operating conditions, the desirable location of the probes with respect to the outlet guide vanes is also dependent upon engine operating conditions. Hence, proper positioning of the P.sub.t probes for sensing pressure behind the outlet guide vane discharge low pressure region cannot be accomplished with certainty.
It is therefore an object of this invention to accurately maintain the fan operating line of a turbofan engine.
Another object of this invention is to provide a means of accurately sensing the fan discharge total pressure in a fan duct having a plurality of outlet guide vanes.
Another object of this invention is to provide in a turbofan engine a means of sensing the fan discharge total pressure without introducing errors caused by the wake of outlet guide vanes within the duct.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.